Abstract

Evidence for the specificity of the radioimmunoassay for morphine in plasma was provided by demonstrating similar results with a fluorometric assay method which did not measure morphine glucuronide. Specificity studies are also presented for a radioimmunoassay for morphine in the brain which is described and has a sensitivity of 100 pg. No differences were observed in the 24-hour plasma disposition of morphine in rats of different sex, nor did quinine or previous exposure to and dependence on morphine alter its plasma disposition. After a single dose of morphine, the T_½ in plasma was 1 to 2 hours for the first 6 hours whereas the T_½ was 10 hours during the 6- to 48-hour time interval. However, morphine was detectable in the plasma for 20 days after the exposure to and removal of morphine pellets. The plasma half-life for this slowly decaying morphine was 7 to 9 days. The disposition of morphine was altered in old rats. Both the serum and brain levels of morphine were markedly higher in 20-month-old rats compared to 2-month-old rats.